OP-160_15 Datasheet, PDF (19/24 Pages) Analog Devices – High-Speed, Current Feedback Operational Amplifier

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OP-160
closed-loop response. For large noninverting gains, R1 is small,
creating a very high-frequency open-loop pole which has limitedef-
USING CURRENT FEEDBACK OP AMPS IN INTEGRATOR
APPLICATIONS
fect on the closed-loop response. As the non inverting gain is de-
The small-signal model of a current feedback op amp shown
creased, R, becomes larger and the stray zero becomes lower in
frequency, having a much greatereffectontheclosed-Ioop response.
earlier in Figure 3 assumes a non-varying value of feedback
impedance- A non-varying feedback impedance ensures that
To reduce peaking at low non inverting gains, place a series resis-
the bandwidth of the amplifier does not extend beyond its 1800
tor, Rc' in series with the non inverting input as shown in Figure 20.
phase shift point and create unwanted oscillations. In integrator
This resistor combines withthe stray capacitance at the noninverting
circuits, the feedback element is a capacitor whose impedance
inputto form a low-pass fifterthat willreduce the peaking. The value
does vary with frequency. By definition then, integrator applica-
of Rc should be determined experimentally in the actual PCB lay-
out. Less peaking willoccur in inverting gain configurations since
tions using current feedback amplifiers should be unstable.
However, a simple trick, shown in Figure 22, enables high-
the inverting input is a virtual ground which forces a constant volt-
speed, wide bandwidth current feedback op amps to be used in
age across the stray capacitance.
integrator applications.
A common practice to stabilize voltage feedback op amps is to use
Resistor RF is placed between an artificial sum node and the
a capacitor across the feedback resistance. This creates a zero in
the voltage feedback amplifier response to offset the loss of phase
margin due to a parasitic pole. In current feedback amplifiers, this
technique willcause the amplifier to become unstable because the
closed-loop bandwidth will increase beyond the stable operating
frequency.
OINVERTING AMPLIFIER
The OP-160 is also capable of operation as an inverting amplifier
B (see Figure 21). The transfer function of this circuit is identical to
that using a voltage feedback op amp:
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inverting input of the amplifier. This resistor maintains a mini-
mum value of feedback impedance over all frequencies. At high
signal frequencies, the integrator capacitor, C" is a short circuit;
the feedback impedance is equal to RF only and the amplifier
has maximum bandwidth. At low frequencies, C, adds to the
overall feedback impedance. This lowers the amplifier's band-
width but not enough 10 affect the integrator's performance.
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FIGURE 22: An Integrator Using a Current Feedback Op Amp
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FIGURE 21: The OP-160 as an inverting amplifier.
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